Peter,
AppBuilder from Intel is a design engineer's best friend. It allows you to have all information on the 8051 and its Intel derivatives on you desktop.
It links to the manual, allows you to configure registers, serial port, etc and will give you code for the hardware setup in both assembly and C.
It is not a substitution for knowledge but a great reference at hand. It includes a section for the assembly instructions and number of bytes and opcodes for each.
I use it often so that I:
1) Do not have to commit every part of the 8051 to memory
and
2) Don't have to have books sprawled out on my desk due to number 1.

Try it, I think you will find it helpful.
Allen